This invention relates to a rotor shroud assembly in a gas turbine engine. In particular, it concerns the control of the clearance between the tips of the rotor blades of a turbine rotor and the encircling shroud assembly.
The radial growth of a bladed turbine rotor disc at any point in an engine operating cycle is governed by three factors namely:
The thermal growth of the rotor disc, which is influenced by the temperature of the high pressure compressor delivery cooling air; PA1 The thermal growth of the turbine blades, which is influenced by the temperature of the combustion gases; and PA1 The centrifugal growth of a complete bladed rotor disc. PA1 a plurality of arcuate shroud liner segments encircling the rotor assembly, each segment being mounted for radial movement and has a radially inner surface spaced from the tips of the blades by a pre-determined clearance, PA1 a first control ring having a relatively rapid radial response to thermal change, PA1 a second control ring having a relatively slow radial response to thermal change, PA1 a mounting device coupled with the first and second control rings and supporting each of the shroud segments such that the radial position of each segment is continuously controlled by the thermal expansion of the first and second control rings in combination.
As a result of engine accelerations, blade thermal growth and bladed rotor disc growth factors respond very quickly. The disc thermal growth factor responds more slowly because of the greater bulk of the disc relative to that of the rotor blades.
These various growth changes affect the clearance between the tips of the rotor blades and the shroud surrounding those blades, and it is important for the purpose of engine operating efficiency that this clearance be controlled at all stages of engine operation.
It is conventional practice to surround the bladed rotor disc with a segmented shroud liner ring having an internal diameter slightly larger than the outside diameter of the blades of the disc so that a small clearance exists between the liner ring and the blade tips. The shroud liner ring comprises a number of segments each of which may change its radial position relative to the adjacent segments. When the engine is running, the liner segment is subject to the same high temperature exhaust gases as pass over the turbine blades so, as the blades change their length, and thus the diameter of the rotor changes, the ring of liner segments also changes its diameter.
It is relatively easy to control the turbine shroud liner segments by means of a control ring so that the shroud liner segments closely follow rotor disc growth at engine steady state conditions. As the major part of the bladed rotor growth is attributed to disc thermal expansion, the control ring is required to have a similarly slow response. However, having matched the control ring with the bladed rotor, problems arise when rapid acceleration and deceleration take place. To follow, as closely as possible, bladed rotor tip movement, the control ring growth must be boosted at the early stages of the acceleration cycle and attenuated at the early stages of the deceleration cycle.